Niploading of SymSizers - problems, corrections and maintenance

Nip loading varies on film coated size presses depending on type of size/coating that is being applied. When applying starch, nip loads range from 100-180 pli. For applying coating, nip loads range from 80-120 pli. Determining where to run the nip load level depends on the crown of the rolls and the incoming sheet.

For every set of roll crowns, theoretically there is one nip load that results in a flat nip load profile. Higher crowns require higher nip loading, and lower crowns require less nip loading for a flat nip load profile. Given a certain set of roll crowns, the incoming sheet determines where the actual nip load needs to be set.

Common problems and corrections

A common problem with the incoming web is wrinkles on the edges or in the center. Wherever the wrinkles are seen, the nip speed in those areas is lower and needs to be increased. Increasing nip load in those areas increases the nip speed. If wrinkles are seen on both edges, increase the overall nip loading. If wrinkles are seen only on one end, skew the nip loading to add more pressure on the end that is wrinkling. When skewing the roll nip pressure, keep the actual amount of skewing below 25 pli.

If skewing is required all the time, the root cause of the skewing should be determined. Factors that may cause nip load skewing include pressure transmitter calibration differences, incorrect drive shaft compensation, nip load pivot pins binding, movable roll applicator beam force synchronizing equipment not working properly, etc.

Periodic maintenance

Finally, it is a good idea to periodically conduct e-nip nip profile measurements to establish what the nip load profiles look like and what nip load setting is required for a flat nip profile. Remember to load the applicator beams when doing the e-nips to show the actual nip load profile under running conditions.

Contact your Valmet representative for more information about resolving nip load problems or to schedule an e-nip test.